1. Field of the Invention
The present invention relates to compositions based on calcium sulfate hemihydrate and, in particular, to such compositions for the production of cores and molds that can be subsequently washed-out with water, to a method of producing such cores and molds, and to the use of such cores and molds for the production of molded articles and molded structural components (hereinafter referred to as "molded articles" but intended to encompass molded articles and molded structural components) made of, for example, composite substances, plastics including thermoplastics, duroplastics, and elastomers, or low melting point metal alloys.
2. Description of the Related Art
Compositions for the production of molds that can be washed-out, especially cores, have been known for a long time. For example, eutectic mixtures are available whose melting temperature lies below 300.degree. C. and, in particular, below 150.degree. C.. These eutectic mixtures are melted in complicated melting vessels equipped with stirring mechanisms and are thereafter cast in molds for the production of cores, winding mandrels and the like. Since, moreover, these eutectic mixtures are sensitive to humidity, it is necessary to take protective measures, such as employing dehumidifiers, in addition to employing expensive melting devices and complicated mixing apparatus.
Low melting point metal alloys are also available for the production of lost cores or molds. These alloys can be, for example, melted out by the application of heat and are thus also suitable for those molds and cores that are required for the production of complicated structural components, e.g., those having internally undercut construction, and molded articles. However, this process also requires expensive melting and casting equipment, as well as temperature resistant molds.
Finally there is the wax melting process which is also unsuitable for many purposes as is well known.
Although natural gypsum is also used for the production of cores and molds, it requires a hammer and chisel to remove such cores and molds from the component since natural gypsum cannot be washed-out with either cold or hot water.